This invention relates to a process and apparatus for dispensing a particulate solid. More specifically, it relates to process and apparatus in which a particulate solid is dispensed on to articles in such a manner that a very uniform distribution of the particulate solid over the surfaces of the articles is obtained.
In many material handling processes, it is necessary to coat articles with a particulate solid in such a manner that the articles receive a uniform amount of the particulate solid per unit area. For example, in food processing it may be desired to treat dough or baked goods with particulate solids such as salt (sodium chloride), cheese powder, sesame seeds or sunflower seeds.
One situation in which very uniform dispensing of a particulate solid on to the surface of articles is of great importance is the addition of so-called "topping salt" to cracker doughs. In the production of crackers, a dough is rolled into thin sheets. These thin sheets are then either cut into the appropriate dough shapes, if round or similarly-shaped crackers are desired, or are indented with the shapes of square or retangular crackers, so that the individual crackers can easily be separated from the sheet after baking. Immediately before baking, salt is sprinkled on to the upper surface of the dough to provide the desired taste in the final crackers. Uniform distribution of the salt is desired in order to avoid variations in the taste of the crackers. Moreover, because of the association of sodium with hypertension, many people are now restricting their salt intake and food manufactures are providing a variety of low sodium products, including low sodium crackers. Government regulations either have, or in the future are likely to, limit the amount of sodium which can be incorporated in low sodium foodstuffs and require specification on the labels of the quantity of sodium of each serving of the foodstuff. If the salt is not spread uniformly over the cracker dough, certain crackers may have sodium contents which exceed the allowable maximum and which will differ in their sodium content from the amounts specified on the label; either problem will subject the food manufacturer to legal penalties.
The uniform dispensing of salt on to cracker dough is made particularly difficult by the large size and high speeds of modern bakery equipment. For example, typical commercial equipment for producing crackers produces belts of dough 36 inches (914 mm.) wide traveling at speeds about 60 to about 180 feet (18.3 to 54.9 m.) per minute. With these wide belts of dough and high rates of dough movement, any momentary fluctuations in amount or lateral distribution of salt across the belt of dough will produce very pronounced variations in the amounts of salt on particular crackers.
Various attempts have been made to provide apparatus which will uniformly distribute salt over a belt of dough. In one form of such apparatus, a stream of salt is simply deposited on a wide sloping plate which is intended to distribute the salt over the width of the belt. Such apparatus fails to ensure uniform distribution of the salt; almost inevitably, the center of the belt of dough receives more salt per unit area than the side edges.
A more sophisticated salt spreader is manufactured by Fedco Systems Inc., 2170 Gunn Highway, Odessa, Florida 33556. In the Fedco salter, a hopper feeds salt on to a roller which has grooves cut around its circumference. A seal is in contact with the roller where it leaves the hopper. Salt from the hopper falls into the grooves and excess salt is wiped from the roller by the seal, which acts in the same manner as a doctor blade. As the roller rotates, it comes into contact with a plow which fits into the grooves and releases the salt on to the belt of dough traveling below it.
Another type of salter manufactured by Heat & Control Inc., 225 Shaw Road, South San Francisco, California 94080 feeds salt from a hopper on to a conveyor belt, which is formed from a metal mesh. A plate is disposed under a portion of the mesh belt to prevent flow of salt therethrough. The salt enters into the apertures in the mesh belt and, as the belt leaves the plate, the salt falls from the upper land of the belt through the lower land on to the product.
A further type of apparatus for dispensing particulate solids sold commercially under the name Sivo-O-Dust by Christy Machine Company, 118 Birchard Avenue, P.O. Box 32, Fremont, Ohio 43420 operates in a manner similar to the Fedco salter discussed above in that the salt is fed from a hopper on to a grooved drum. However, in the Siva salter the grooves on the drum extend axially along the drum and are of substantially triangular cross-section tapering inwardly. As the roller passes through the hopper, the grooves fill with salt and as the drum turns the salt is released near the bottom of the drum on to the belt which carries the dough beneath the drum.
The Fedco, Heat & Control and Siv-O-Dust salters all suffer from the disadvantage that they are essentially constant-volume rather than constant-weight salters, and the bulk density of salt can vary considerably depending upon crystal size and form, moisture content and other factors. Consequently, in practice all of these salters are subject to variations in weight of salt dispensed per unit area of approximately 5 percent, a variation which leads to problems when one is salting low sodium crackers.
Vibrating conveyors, in which a particulate solid is caused to move along a surface by vibrating the surface, are used in various industries for conveying particulate solids. We have been made attempts to use a form of vibratory feeder in which salt is allowed to pass over the edge of a vibrating plate to spread salt over a belt of dough, in the hope that the salt would spread uniformly across the vibrating plate and produce a uniform dispersion of salt on the belt of dough. In practice, however, it has been found that simply allowing salt to flow off one edge of a vibrating plate does not produce uniform distribution of salt across the width of a wide belt such as those used in commercial cracker manufacturing equipment.
Various systems for delivering a particulate solid at a constant weight per unit time are known. For example, U.S. Pat. Nos. 3,139,216 and 3,139,217, both to Mell, describe an apparatus in which a particulate solid flows from a hopper on to a conveyor belt. A weight sensing device is dispensed below the conveyor belt and measures the weight of the particulate solid on the conveyor belt. An electrically-operated control system receives signals from the sensing device and signals indicating the speed of the conveyor belt, and adjusts the speed in order to ensure that the particulate solid is discharged from the conveyor belt at a constant weight per unit time.
U.S. Pat. No. 2,622,766 to Simon describes an apparatus rather similar to that described in the Mell patents; this apparatus includes a conveyor belt, a means for feeding a particulate solid on to the conveyor belt and a weight sensing means sensitive to the weight of the material on the conveyor. The output from the weight sensing means is supplied to a control system, which either varies the speed of the conveyor belt, or varies the rate of operation of the feeding apparatus, so that a controlled amount of bulk material is supplied at a constant rate. When the conveyor belt speed is fixed, the adjustable feeding mechanism may include either a feeding aperture of variable size, or a variable-speed auger.
U.S. Pat. Nos. 2,637,434 to Harper, 2,917,207 to Prowse et al., 3,362,585 to Nessim et al., and 3,110,419 to Atkins et al., all describe various types of feeding apparatus in which a particulate solid flows from a hopper through an aperture on to a belt, and in which a load sensor is provided to measure the weight of particulate solid on the belt. The size of the aperture through which the particulate solid leaves the hopper is adjusted in response to the weight of material sensed by the weight sensor.
None of the aforementioned patents describe any mechanism for distributing the steady stream of particulate solid which they produce uniformly across a discharge opening of substantial width.
It will be seen that there is thus a need for a process and apparatus for uniformly distributing a particulate solid over articles which is capable of spreading the particulate solid uniformly across an article or articles of substantial width, and which does not depend upon measuring the volume of the solid being dispensed. This invention provides such a process and apparatus.